1. Field of the Invention
This invention relates to a half-mirror made of a synthetic resin.
2. Related Art Statement
Heretofore, a half-mirror has been manufactured by alternately depositing a TiO.sub.2 layer and a SiO.sub.2 layer, for example, onto a heated glass substrate under vacuum through an electron gun process.
Lately, synthetic resins having a light weight and an easy workability have frequently been used as a material for optical parts such as lens and like instead of the glass. However, the optical parts made from the synthetic resin are apt to be damaged because the surface is soft, so that it is required to form a cured protective film layer on the surface of the optical part. Among the optical parts, the half-mirror is produced by forming a semi-permeable reflective film layer on the surface of the substrate made from the synthetic resin and is used in a camera, spectrometer, light interference film pressure meter or the like. In this case, it is necessary to form a reflection preventive film, reflection increase film, semi-permeable film or the like on the surface of the half-mirror.
These films are generally formed by a vacuum deposition process, but when the substrate is made from the synthetic resin, it is impossible to form the deposited film on the substrate by forcedly heating the substrate as in the case of a glass substrate, because the synthetic resin substrate itself is degraded and decomposed by heating. If the synthetic resin substrate is subjected to a deposition process without heating, since a greater part of the depositing materials used for glass are dielectric substances capable of forming film by heating such as oxides, fluorides and the like, micro cracks are produced in the resulting deposited film after the deposition, which is particularly remarkable in case of acrylic resins or the like for injection molding. Moreover, even if abnormal phenomenon is not observed in the deposited film, micro cracks occur during the thermal shock test of the optical part after the deposition. Particularly, if it is intended to form a TiO.sub.2 film on the substrate, since radiant heat is high, the synthetic resin substrate is degraded or deformed by vacuum deposition over a long period of time and consequently the use of the substrate itself is impossible.
Heretofore, the half-mirror made from the synthetic resin produced by forming the film on the surface of the synthetic resin substrate without heating the substrate has been proposed, for example, in Japanese patent laid open No. 60-225101 and 61-110101.
The synthetic resin half-mirror described in Japanese Patent laid open No. 60-225,101 is produced by forming silicon monooxide (SiO) film on the surface of the substrate made of acrylic resin through reaction deposition of oxygen (O.sub.2) and then alternately laminating a ditantalum penta oxide (Ta.sub.2 O.sub.5) film and a silicon dioxide (SiO.sub.2) films thereon. On the other hand, the synthetic resin half-mirror described in Japanese Patent laid open No. 61-110,101 is produced by forming a primer and hard coat film (thickness: 5 .mu.g/m.sup.2) of a silicon paint on the surface of the substrate made of methacrylic resin and further forming a dielectric thin film of titanium dioxide (TiO.sub.2) thereon through reactive sputtering of Ti metal as a target.
In the half-mirror of Japanese Patent laid open No. 60-225,101, however, micro cracks occur in the film after the thermal shock test of repeating a temperature cycle of -30.degree. C. room temperature (20.degree..about.25.degree. C.).fwdarw.80.degree. C. ten times. Furthermore, since Ta.sub.2 O.sub.5 is high in the melting point, when the vacuum deposition is performed by an electron gun process, the splushing (adhesion of Ta.sub.2 O.sub.5 in form of granules, resulting in the falling off from the film) is apt to be caused and there is a problem in the resulting appearance. Furthermore, the synthetic resin substrate is not heated so as to prevent the degradation and decomposition of the substrate, but it is influenced by a radiant heat. Therefore, it is required to be a long distance from the deposition source for avoiding the influence of the radiant heat and also to perform the cooling of every layer deposited without continuously forming multilayer films through vacuum deposition, resulting in the reconstruction of the device, the decrease of working efficiency and the like.
On the other hand, in the half-mirror described in Japanese Patent laid open No. 61-110,101, TiO.sub.2 film is formed on the primer and hard layer through the sputtering of Ti metal, so that it is difficult to hold the surface shape having an optical accuracy, particularly a lens shape, after the film formation due to the presence of the undercoat.